Endoscopes as known from prior art, for example, for minimally invasive surgery, guide an image by means of rod lenses from an intracorporeal objective lens to an extracorporeal occular. Due to the rod lenses, the system is rigid and limited in optical quality. Modern video endoscopes use a camera chip in the endoscope tip. Such an endoscope is disclosed in U.S. Pat. No. 7,365,768 B1. This has a rigidly arranged lens in front of the camera chip. An adjustment of the focal length of the lens is not possible.
DE 103 23 629 A1 discloses a moving field linear motor which includes at least three stator coils. A phase-shifted current supply to the coils produces a moving field which effects a displacement of the armature with axial permanent magnets.
From DE 10 2008 038 926 A1, a linear drive including two axially polarized permanent magnets in an armature is known. The armature is deflected in axial direction by the current supply to the stator coils. Additionally, stable positions of the armature are realized by pole shoes mounted to the stator, allowing a continuous displacement of the armature in a guiding tube.
In DE 10 2010 000 582 A1, a further linear drive is disclosed, which has an axially polarized permanent magnet in the armature, and one or two axially polarized permanent magnets in the stator.
These three linear drives each include a stator and an armature. The armatures are construed of one or several permanent magnets. For deflection and for generation of the electromagnetic flux in defined directions, rings of soft magnetic iron are disposed at the permanent magnets (pole shoes). One or more coils generate Lorentz-forces in the stator. Additional permanent magnets and rings of soft magnetic iron serve partially for generation of reluctance forces. The stator is enclosed by a soft-magnetic sleeve which constitutes a reflux yoke for the magnetic flux. In a current less state, the armature is in a so-called idle position due to resetting reluctance forces. Due to supplying the coils with electric current of constant current strength, Lorentz-forces are generated which lead to a continuous deflection of the armature from the rest position. By generating a force balance of Lorentz-forces in the DE 103 23 629 A1, or of Lorentz-forces and reluctance forces in DE 10 2008 038 926 A1 and DE 10 2010 000 582 A1, the armature stays in a deflected position. Thereby it is preferred to have a predetermined relationship between the magnitudes of currents to positions of the armature. Normally, this in achieved by calibration after setup of the drives.
External forces which are difficult to control, such as for example friction forces or gravity, lead to positioning inaccuracy. By predefining fixed magnitudes of current, the position of the armature can be determined only in a limited manner.
DE 196 05 413 A1 discloses a linear drive with position measurement. Here, the drive winding is at the same time used as measurement winding. By such a position measurement, a higher positioning accuracy can be achieved. However, the preciseness of the measurement system and thus of the control system is limited due to the minor change of the coil inductivity during movement of the armature.
In U.S. Pat. No. 5,747,952, a three-phase linear drive is disclosed, where the magnetic field is measured by a Hall sensor between coil and armature, and the amplitude of the control signal is held at a constant value.